The plan is a peaking of effort at Lawrence Livermore National Labs National Ignition Facility from decades of gathering experience. The trigger for the news is indirect-drive hohlraum (the tiny cylindrical target which contains an even tinier spherical fuel sphere filled with deuterium and tritium the two isotopes of hydrogen) experiments at the National Ignition Facility have demonstrated symmetric capsule implosions at unprecedented laser drive energies of 0.7 Mega Joules.

National Ignition Facility Instrumentation 'Dante' Click image for more info.

That is they’ve hit the target well enough that it implodes instead of whizzing off somewhere, using more energy than past attempts and figured out how to use the laser concentration complications to their advantage. These experiments are another form of the now familiar term – inertial confinement fusion (ICF).

The one hundred and ninety-two simultaneously fired laser beams heat “ignition emulate hohlraums” to radiation temperatures of 3.3 million degrees Kelvin, compressing 1.8-mm capsules by the soft x-rays produced by the photon struck hohlraum. The very quick compression of the fuel capsule forces the hydrogen nuclei to combine, or fuse, releasing many times more energy than the laser energy that was required to spark the reaction.

The problem for decades has been because of the tendency of the laser beams to scatter and dissipate their energy. The last reported series of test shots using helium and hydrogen filled targets last fall, NIF researchers were able to use laser-plasma interactions, or LPI, (those complications mentioned earlier) effects to their advantage and to adjust the energy distribution of NIF’s laser beams.

ICF Program Director Brian MacGowan explains, “Laser-plasma interactions are an instability, and in many cases they can surprise you. However, we showed in the experiments that we could use laser-plasma interactions to transfer energy and actually control symmetry in the hohlraum. Overall, we didn’t find any pathological problem with laser-plasma interactions that would prevent us generating a hohlraum suitable for ignition.”

Siegfried Glenzer, NIF plasma physics group leader takes the explanation further. Using LPI effects to tune ICF laser energy is “a very elegant way to do it. You can change the laser wavelengths and get the power where it’s needed without increasing the power of individual beams. This way you can make maximum use of all the available laser beam energy.” That cylinder holding the fuel is understood to be very important now.

In a Science Express article, Glenzer, MacGowan and their NIF colleagues report “self-generated plasma-optics gratings on either end of the hohlraum tune the laser power distribution in the hohlraum, producing symmetric X-ray drive.” Glenzer said the gratings act like tiny prisms, redirecting the energy of some of the laser beams just as a prism splits and redirects sunlight according to its wavelength.

Glenzer attributes the newly understood LPI phenomenon to the size of the test hohlraums that are somewhat smaller than actual NIF ignition targets. They’re two to three times larger than hohlraums used in previous ICF experiments at other laser facilities. He said the increased amount of the high-temperature, low-density plasma in the areas where the laser beams enter the hohlraum was responsible for the spontaneous generation of the plasma gratings.

The second aspect is the technique of slightly shifting the wavelength of some laser beams to control the transfer of energy between the beams and equalize the laser power distribution in the hohlraum. This is a result of predictions modeled by NIF scientists using high-fidelity three-dimensional simulations. In last fall’s experiments, an initially asymmetric target (the cylinder) implosion into a “pancake” shape was changed to a spherical shape by the wavelength-shifting technique, validating the modeling results.

Glenzer explains further, by taking advantage of the LPI effects in the target, as the beams crossed at the entrance of the hohlraums, the scientists could make use of minute wavelength adjustments, ranging from a fraction of an angstrom to a few angstroms (an angstrom is one ten-billionth of a meter, about the size of an atom). With the LPI scheme Glenzer says, “you can run every beam at maximum power and have another distribution mechanism to achieve symmetry.” Sounds rather elegant now,

What is happening from the old idea of trying to get photons to compress and heat a fuel, the container is bombarded with photons such that it emits X-rays to compress and heat the fuel.

Jeff Atherton, director of NIF experiments says, “We feel we will be able to create the necessary hohlraum conditions to drive an implosion to ignition.” That notion is from extrapolating the results of the earlier experiments to higher-energy shots on full-sized hohlraums.

The National Ignition Facility’s next step is to move to ignition-like fuel capsules that require the fuel to be in a frozen hydrogen layer (at 425 degrees Fahrenheit below zero) inside the fuel capsule. The NIF is currently being made ready to begin experiments with ignition-like fuel capsules in the summer of 2010.